Many human subjects develop an unstable shoulder joint, usually as a result of injury, leading to symptoms which include pain, limited range of motion, and recurrent dislocation of the joint. For diagnostic purposes, and to assist in the assessment of the outcomes of various surgical interventions made in an effort to improve the stability of the joint, it is desirable to clinically evaluate the stability of the shoulder joint.
In the prior art, to clinically evaluate the stability of the shoulder joint, it has been common practice for the clinician to subjectively estimate the relative displacement of one bone of the joint (e.g. the humerus) with respect to another bone of the joint (e.g. the glenoid scapula) when a "relocation force" is applied near the head of the humerus toward the glenoid scapula, at specific positions and orientations of the joint. Also, many clinicians manipulate the humerus to note the range of motion possible in a symptomatic shoulder, and stop or limit these manipulations when the subject exhibits significant "apprehension", experiences pain, or actually experiences a dislocated shoulder joint. Contrary to clinical practice in the prior art, the inventors have discovered as a result of an initial clinical investigation that relative displacement of the humerus with respect to the glenoid scapula may not be consistently and significantly larger in a symptomatic shoulder than in a normal shoulder, and thus relative displacement may not be as clinically useful as previously thought in assessing shoulder joint stability for diagnostic purposes or to assist in the assessment of the outcomes of surgical interventions.
Also, in the prior art, clinical determination of the position and orientation of the bones forming the joint is subjective, imprecise and therefore difficult or impossible to reproduce for serial evaluations of changes in shoulder joint stability in the same subject. For example, in the prior art, clinicians have typically used a simple hand-held goniometer to estimate the orientation of the humerus relative to the torso of the subject. Also, in such prior-art determinations, the rate of change of the position and orientation of the humerus relative to the glenoid scapula, as well as the level of force applied near the head of the humerus, have been assessed only subjectively if at all by clinicians.
In the prior art, patient "apprehension" has been defined subjectively and clinical estimates of a patient's level of apprehension vary widely among even experienced clinicians. In the prior art, in determining the stability of the shoulder joint, the clinician may observe the apprehension exhibited by the patient as the humerus of the patient is manipulated through a desired range of motion relative to the glenoid scapula and force is applied near the head of the humerus. During those manipulations, apprehension is subjectively defined as the patient response to an oncoming feeling that the patient's shoulder joint is in a position of instability and may dislocate if the position and orientation of the bones comprising the joint is not changed to a stable position. Patient responses include a look or feeling of apprehension or alarm on the patient's face, resistance to further motion in the direction of instability, and verbally stating that this position duplicates the feeling of a previous dislocation. Thus in the prior art, an objective, quantitative indication of apprehension is not available to the clinician.
Finally, in the prior art, evaluation of changes in the stability of the shoulder joint over time, for use in patient outcome studies, has been dependent on the accuracy of a specific clinician's recorded subjective observations, and often on consistency in recorded observations among different clinicians. Furthermore, in the prior art it has not been common for any clinician to accurately measure or record the orientation of the arm during the test, thereby introducing a major variable and source of error into any serial comparisons.